There's not really an 'appropriate' level. You need UC to be as high as you can get it; I always used to train UC the same way I would a DD spell, just pump it as high as it can go and boost it as much as you can with Helms, Gi and elven gear. But you also need your guy to be strong and fast; so you need as much strength and dex as you can manage. No point in a wimpy tank, it doesn't matter how much you can dodge your opponents if you can't kill them.....

I don't believe there is an "ideal ratio" or even anywhere close to an approximate...

Long version:

UC takes a lot of XP to really go anywhere. When ya first first start off, I would advise a base UC, relying on your HGs and/or Gi for boosting. This will allow you to get your HP/ST/DX to decent levels when you're still low enough for 1k in a stat to make a difference ;) Once the levels of your opponents become more spread apart, start pumping heavy HP and UC. The effect of UC is it's pth, so by raising that, you are effectively raising your CTH, whereas training DX when you already have the DX advantage gains you nothing, and the UC effect will tend to have a far greater toll on your damage than your ST in the mid levels. UC's inherrant evasion is also wickedly powerful. From there, it's all down to going by feel... See what you'll need for the next group of opponents, and train that, I guess...